In image compression methods such as Motion Picture Experts Group (MPEG)-1, MPEG-2, MPEG-4 and H.264/MPEG-4 Advanced Video Coding (AVC), one picture is divided into macroblocks to encode an image. Then, the respective macroblocks are encoded using inter prediction or intra prediction.
In intra prediction, a current block of a picture is encoded not using a reference picture, but using values of reconstructed pixels spatially adjacent to the current block. An intra prediction mode with little distortion is selected by comparing a prediction block generated using the adjacent pixel values with an original macroblock. Then, using the selected intra prediction mode and the adjacent pixel values, prediction values of the current block are calculated. Differences between the prediction values and pixels values of the original current block are calculated and then encoded through transform coding, quantization and entropy coding. The intra prediction mode is also encoded.
In 4×4 conventional intra prediction, there are nine modes of a vertical mode, a horizontal mode, a DC mode, a diagonal down-left mode, a diagonal down-right mode, a vertical right mode, a vertical left mode, a horizontal-up mode and a horizontal-down mode.
According to H.264 standard, one mode is selected among the nine modes to generate a prediction block of the current block. According to HEVC standard under development, there are 17 or 34 intra prediction modes.
However, when some or all values of pixels adjacent to current block do not exist or are not already encoded, it is impossible to apply some or all of the intra prediction modes to the current block. Also, when there is a large difference between adjacent reference pixels, distortion between a prediction block and an original block becomes large. Therefore, the coding efficiency is degraded.
Also, as the number of intra prediction modes increases, a new method of encoding the intra prediction mode of the current block to minimize the amount of coding bits, and a more effective method of decoding the intra prediction mode and generating a reconstructed block are required.